US 20040190640 A1 Abstract The present invention divides the available sub-carriers in an OFDM symbol window into N groups of sub-carriers wherein each group will be associated with sub-bands. In one embodiment, the sub-carriers in a group are spread throughout the range of sub-carriers to improve frequency diversity, and the sub-carriers assigned to any one group are separated by a constant offset. The sub-carriers in the group may be offset by an integer power of two. Within each group, sub-bands are defined using frequency hopping patterns among sub-carriers in the group from one OFDM symbol window to another. A pseudo-random pattern may be employed for sub-carrier mapping from one OFDM symbol window to the next to effectively distribute the sub-bands across the selected band of sub-carriers for the group.
Claims(64) 1. A method for communicating in an orthogonal frequency division multiplexing (OFDM) environment comprising:
a) associating traffic to be transmitted to a plurality of user elements with corresponding sub-bands, each sub-band defined by a sequence of sub-carriers over a plurality of OFDM symbol windows, the sub-carriers for each sub-band associated with one of a plurality of groups of sub-carriers within an OFDM frequency band; b) mapping the traffic into quadrature-based symbols; c) for each of the user elements, encoding the quadrature-based symbols onto the sub-carriers for the sub-band associated with the user element; and d) modulating the sub-carriers using an Inverse Fast Fourier Transform to create OFDM symbols for transmission. 2. The method of 3. The method of 4. The method of 5. The method of ^{x},wherein x is an integer. 6. The method of 7. The method of 8. The method of 9. The method of 10. The method of 11. The method of 12. The method of 13. The method of 14. The method of 15. The method of 16. The method of 17. A method for communicating in an orthogonal frequency division multiplexing (OFDM) environment comprising:
a) receiving OFDM symbols, the OFDM symbols carrying traffic for a user element in a sub-band defined by a sequence of sub-carriers over a plurality of OFDM symbol windows, the sub-carriers for the sub-band associated with one of a plurality of groups of sub-carriers within an OFDM frequency band; b) demodulating the OFDM symbols using a Fourier Transform to recover sub-carriers encoded with quadrature-based symbols; and c) decoding the quadrature-based symbols encoded onto the sub-carriers to recover the traffic for the user element. 18. The method of 19. The method of 20. The method of 21. The method of ^{x},wherein x is an integer. 22. The method of 23. The method of 24. The method of 25. The method of 26. The method of 27. The method of 28. The method of 29. The method of 30. The method of 31. The method of 32. The method of 33. A system for communicating in an orthogonal frequency division multiplexing (OFDM) environment comprising:
a) wireless communication electronics; and b) a control system associated with the wireless communication electronics and adapted to:
i) associate traffic to be transmitted to a plurality of user elements with corresponding sub-bands, each sub-band defined by a sequence of sub-carriers over a plurality of OFDM symbol windows, the sub-carriers for each sub-band associated with one of a plurality of groups of sub-carriers within an OFDM frequency band;
ii) map the traffic into quadrature-based symbols;
iii) for each of the user elements, encode the quadrature-based symbols onto the sub-carriers for the sub-band associated with the user element; and
iv) modulate the sub-carriers using an Inverse Fourier Transform to create OFDM symbols for transmission.
34. The system of 35. The system of 36. The system of 37. The system of ^{x},wherein x is an integer. 38. The system of 39. The system of 40. The method of 41. The system of 42. The system of 43. The system of 44. The system of 45. The system of 46. The system of 47. The system of 48. The system of 49. A user element for communicating in an orthogonal frequency division multiplexing (OFDM) environment comprising:
a) receiving OFDM symbols, the OFDM symbols carrying traffic for a user element in a sub-band defined by a sequence of sub-carriers over a plurality of OFDM symbol windows, the sub-carriers for the sub-band associated with one of a plurality of groups of sub-carriers within an OFDM frequency band; b) demodulating the OFDM symbols using a Fourier Transform to recover sub-carriers encoded with quadrature-based symbols; and c) decoding the quadrature-based symbols encoded onto the sub-carriers to recover the traffic for the user element. 50. The user element of 51. The user element of 52. The user element of 53. The user element of ^{x},wherein x is an integer. 54. The user element of 55. The user element of 56. The user element of 57. The user element of 58. The user element of 59. The user element of 60. The user element of 61. The user element of 62. The user element of 63. The user element of 64. The user element of Description [0001] This application claims the benefit of U.S. provisional application serial No. 60/451,127, filed Feb. 28, 2003, the disclosure of which is incorporated herein by reference in its entirety. [0002] The present invention relates to wireless communications, and in particular to allocating sub-carriers in an orthogonal frequency division multiplexing system. [0003] Since orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique, the available spectrum is divided into many sub-carriers, each being modulated by data at a relatively low data rate. OFDM can support multiple access by allocating different sub-carriers to different users. The sub-carriers for OFDM are orthogonal and closely spaced to provide an efficient spectrum. Each narrow band sub-carrier is modulated using various modulation formats, such as quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM). OFDM modulation is provided using an Inverse Fast Fourier Transform (IFFT). Initially, data for transmission is mapped into quadrature-based symbols that are encoded onto the individual sub-carriers. An IFFT is performed on the set of modulated sub-carriers to produce an OFDM symbol in the time domain. Typically, a cyclic prefix is created and appended to the beginning of the OFDM symbol before it is amplified and transmitted. During reception, the OFDM symbols are processed using a fast Fourier transform (FFT) to recover the modulated sub-carriers, from which the transmitted symbols can be recovered and decoded to arrive at the transmitted data. [0004] As noted, to facilitate multiple user access, data for transmission is allocated to groups of adjacent sub-carriers, wherein these groups remain consistent from one OFDM symbol to the next. With reference to FIG. 1, each circle represents a sub-carrier for a sequence of OFDM symbols. Each row represents the sub-carriers associated with an OFDM symbol, and each OFDM symbol is transmitted in sequence over time. In this example, users [0005] In an effort to minimize the impact of the variations in the channel, frequency-hopping schemes have been employed to systematically remap the groups of sub-carriers associated with each user to different points in the time-frequency plane, as illustrated in FIG. 2. Thus, users are assigned one or more transmission blocks consisting of a set number of sub-carriers within a set number of adjacent OFDM symbols. Thus, a user does not necessarily transmit on the same sub-carrier group for every OFDM symbol, but will jump to a different sub-carrier after a period of time based on the defined hopping pattern. The sub-carrier hopping scheme illustrated in FIG. 2 improves the performance over the fixed time-frequency allocation illustrated in FIG. 1; however, the performance could be further improved if the diversity across the whole band were fully exploited. [0006] Most solutions proposed to reduce the interference in frequency-hopped systems are based on the assumption that the different interfering transmitters are synchronized through a global positioning system (GPS) or the like. These solutions are not applicable to communication systems that are not synchronized, such as Universal Mobile Telecommunications System (UMTS). [0007] Other frequency hopping schemes are based on non-synchronized transmitters, but they usually use different pseudo-random hopping sequences, with no way to discriminate the interference level for separate receivers. Hence, a receiver experiencing a low carrier-to-interference ratio will get the same probability of sub-carrier collisions as a receiver with a high carrier-to-interference ratio. This is not optimal, since the high-carrier-to-interference ratio receiver does not necessarily need to avoid collisions as much as a low carrier-to-interference ratio receiver. Thus, there is a need for an efficient sub-carrier mapping technique to minimize the impact of channel variations and interference over the time-frequency plane. [0008] The present invention provides a frequency hopping technique for allocating sub-carriers in an OFDM environment to minimize the impact of channel variations and interference. In general, an OFDM symbol window relates to the time period in which an OFDM symbol is transmitted, and sub-bands are communication channels defined by a sequence of sub-carriers over multiple OFDM symbol windows. A sub-carrier for a given sub-band may hop from one OFDM symbol window to another. Thus, each sub-band is defined by a hopping pattern for sub-carriers over a sequence of OFDM symbol windows. One or more of these sub-bands may be assigned to a user for communications. [0009] From one OFDM symbol window to the next, each sub-band is generally associated with a group of sub-carriers, which may or may not hop from one symbol to the next depending on the mapping scheme for frequency hopping. In operation, data for a given user is associated with one or more sub-bands, depending on the necessary throughput. The allocation of sub-bands to users may dynamically vary depending on the required throughput. [0010] The present invention divides the available sub-carriers in an OFDM symbol into N groups of sub-carriers wherein each group will be associated with sub-bands using the sub-carriers for the group. In one embodiment, the sub-carriers in a group are spread throughout the range of sub-carriers to improve frequency diversity. For maximum frequency diversity, the sub-carriers assigned to any one group are separated by a constant offset. [0011] To minimize the complexity of demodulation using a fast Fourier transform techniques, the sub-carriers in the group are offset by a power of two (2 [0012] Within each group, sub-bands are defined using frequency hopping patterns among sub-carriers in the group from one OFDM symbol window to another. A pseudo-random pattern may be employed for sub-carrier mapping from one OFDM symbol window to the next to effectively distribute the sub-bands across the selected band of sub-carriers for the group. [0013] Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. [0014] The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention. [0015]FIG. 1 is an OFDM time-frequency plan according to one prior art embodiment. [0016]FIG. 2 is an OFDM time-frequency plan according to a second prior art embodiment. [0017]FIG. 3 is a time-frequency plan illustrating frequency hopping. [0018]FIG. 4 illustrates a preferred process for allocating sub-carriers according to one embodiment of the present invention. [0019]FIG. 5 is a sub-band indexing plan according to one embodiment of the present invention. [0020]FIG. 6 is a block representation of a base station according to one embodiment of the present invention. [0021]FIG. 7 is a block representation of a user element according to one embodiment of the present invention. [0022]FIG. 8 is a logical representation of a transmitter according to one embodiment of the present invention. [0023]FIG. 9 is a logical representation of a receiver according to one embodiment of the present invention. [0024] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. [0025] The present invention provides a technique for allocating sub-carriers in an OFDM environment to minimize the impact of channel variations and interference. With reference to FIG. 3, an exemplary time-frequency plan in an OFDM spectrum is illustrated. Each row represents the available sub-carriers associated with a given OFDM symbol being transmitted over an OFDM symbol window. An OFDM symbol window relates to the time period in which an OFDM symbol is transmitted. A sub-band is a communication channel defined by a sequence of sub-carriers over multiple OFDM symbol windows. As illustrated by the darkened circles, a sub-carrier for a given sub-band may hop from one OFDM symbol window to another. Thus, the sub-band is defined by a hopping pattern for sub-carriers over a sequence of OFDM symbol windows. One or more of these sub-bands may be assigned to a user for communications. Although not illustrated, there may be multiple sub-carriers for a given sub-band. [0026] From one OFDM symbol window to the next, each sub-band is generally associated with a group of sub-carriers, which may or may not hop from one symbol to the next depending on the mapping scheme for frequency hopping. In operation, data for a given user is associated with one or more sub-bands, depending on the necessary throughput. The allocation of sub-bands to users may dynamically vary depending on the required throughput. [0027] In the one embodiment, the available sub-carriers in an OFDM symbol window are divided into N groups of sub-carriers wherein the sub-carriers in a group are spread throughout the range of sub-carriers to improve frequency diversity. FIG. 4 illustrates an exemplary scenario where there are 729 sub-carriers indexed as sub-carriers 0 through 728 divided into eight groups (N=8), which are referenced A through H. The sub-carriers for group C are highlighted. For maximum frequency diversity, the sub-carriers assigned to any one group are separated by a constant offset. Further, the sub-carriers for one group will not form part of another group. [0028] To minimize the complexity of demodulation using a fast Fourier transform techniques, the sub-carriers in the group are offset by a power of two (2 [0029] Thus, to maximize frequency diversity and minimize demodulation complexity, the number of groups will be a power of two. In the illustrated example, N=2 [0030] Γ [0031] Γ [0032] Γ [0033] Γ [0034] Γ [0035] Γ [0036] Γ [0037] Γ [0038] Within each group, sub-bands are defined using frequency hopping patterns among sub-carriers in the group from one OFDM symbol window to another. For group C, an example is provided in FIG. 5 wherein a pseudo-random pattern is employed for sub-carrier mapping from one OFDM symbol window to the next to effectively distribute eight sub-bands (C [0039] The number of sub-carriers per sub-band is M and can be fixed or variable. If fixed, then M establishes the number of sub-carriers for each sub-band, and for the above example, M is equal to 729/(N×L)], where L is the number of sub-bands per group and N is the number of groups. For instance, if N=8 and L=7, M=13. [0040] For each group Γ Π [0041] where k is the OFDM symbol (window) number ( [0042] For a fixed value of sub-carriers per sub-band (M), the frequency hopped set of sub-carriers in sub-band S [0043] where Π [0044] For a variable value of M, that is M(i,j), the frequency hopped set of sub-carriers in sub-band S [0045] with [0046] Note that in this case the total number of sub-carriers in sub-band S [0047] In operation, various types of information are transmitted between communicating devices. The information may include pilot signals, control signaling, and traffic, which may represent traditional data, audio, video, or voice. In one embodiment, the pilot signals and control signalling for a sector or cell can be confined to one of the N groups. For a group containing pilot signals and control signals, only the unused sub-carriers may be assigned for the traffic carrying sub-bands. [0048] When assigning groups and sub-bands for communications, various factors may be taken into consideration. For example, the number of sub-carriers and sub-bands may vary on the desired throughput or the time sensitivity of the traffic being transmitted. The allocation of sub-carriers into sub-bands and groups allows enhanced traffic scheduling as well as handoffs from one access point, such as a cellular base station, to another. With regard to scheduling, the various groups defined for the OFDM spectrum may be allocated to different sectors in different ways. For example, certain groups may be used by every sector in every cell, regardless of whether the sectors or cells are adjacent to one another. Other groups can be reserved for select sectors, preferably those that are not adjacent to sectors using the same group. Thus, certain groups will be isolated from one another in the communication environment. For example, mobile terminals communicating with base stations may report channel conditions associated with the data being received back to the base station. These channel conditions may be measured or estimated in a number of ways known to those skilled in the art, and often relate to the carrier-to-interference ratio. Over time, the base stations or an appropriate scheduling entity therefor can gather information on a relatively long-term basis to determine the average channel conditions for each mobile terminal. If the channel conditions are acceptable, the scheduling entity may assign a mobile terminal to a group that is also used in other sectors or cells. In essence, since the channel conditions are acceptable, the mobile terminal is deemed to be able to handle a higher-interference environment and is thus placed in a group that is used by multiple and potentially adjacent sectors or cells. If the channel conditions are poor for a particular user, the scheduler may assign the user to a sub-band in a group that is not used in adjacent sectors or cells. In such a group, there will be less interference since the group is not reused in adjacent sectors or cells, and the channel conditions will invariably improve. This type of scheduling takes place at a relatively slow rate, and will allow users to gravitate towards an acceptable carrier-to-interference ratio. The scheduling of traffic for a particular user within a group may be based on the reported channel conditions for that particular group. The mobile terminal may monitor and report only channel conditions related to the group, and the scheduler may only take into consideration those measurements for scheduling traffic in the group. Accordingly, processing is reduced by only taking into consideration the channel conditions for an associated group. As such, channel conditions for other groups in the given sector or cell do not have to be considered for normal traffic scheduling within a group. Scheduling may be configured to take advantage of the best channel conditions, or may use the channel conditions to assure a certain quality of service for all users within the group. [0049] For handoffs from one sector or cell to another, a user may be assigned a first sub-band in a first group of a first base station, and a second sub-band of a second group for a second base station. The mobile terminal will then communicate during the soft handoff using the first and second sub-bands of the first and second groups until the handoff is complete. Those skilled in the art will recognize additional benefits of the sub-carrier allocation techniques of the present invention. [0050] An exemplary architecture for implementing the above concepts is illustrated below. Those skilled in the art will recognize the various modifications and changes from that described below that are still within the scope of the teachings herein and the claims that follow. [0051] With reference to FIG. 6, a base station [0052] The baseband processor [0053] On the transmit side, the baseband processor [0054] With reference to FIG. 7, a user element [0055] For transmission, the baseband processor [0056] The present operation uses OFDM in a communication system, which may incorporate spatial diversity. OFDM modulation generally relies on the performance of an Inverse Fast Fourier Transform (IFFT) on the symbols to be transmitted. For demodulation, the performance of a Fast Fourier Transform (FFT) on the received signal is used to recover the transmitted symbols. In practice, an Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT) are implemented using digital signal processing for modulation and demodulation, respectively. [0057] In the preferred embodiment, OFDM is used at least for the downlink transmission from the base stations [0058] With reference to FIG. 8, a logical transmission architecture is provided according to one embodiment. In this embodiment, the base station [0059] Bit interleaver logic [0060] If space-time coding (STC) is employed, symbols on each sub-carrier may be presented to optional STC encoder logic [0061] Regardless of STC encoding, the modulated sub-carriers may be selectively directed along a transmission path associated with a desired one of the antennas [0062] Multiplexing logic [0063] After IFFT processing, a cyclic prefix and pilot headers are added to the beginning of the OFDM symbols by prefix and pilot header insertion logic [0064] Reference is now made to FIG. 9. Upon arrival of the transmitted signals at each of the antennas [0065] Regardless of STC decoding, the recovered set of sub-carriers is sent to sub-band de-mapping logic [0066] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. Referenced by
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